ABSTRACT Dissimilatory NO ₃ ⁻ reduction in sediments is often measured in bulk incubations that destroy in situ gradients of controlling factors such as sulfide and oxygen. Additionally, the use of unnaturally high NO ₃ ⁻ concentrations yields potential rather than actual activities of dissimilatory NO ₃ ⁻ reduction. We developed a technique to determine the vertical distribution of the net rates of dissimilatory nitrate reduction to ammonium (DNRA) with minimal physical disturbance in intact sediment cores at millimeter-level resolution. This allows DNRA activity to be directly linked to the microenvironmental conditions in the layer of NO ₃ ⁻ consumption. The water column of the sediment core is amended with ¹⁵ NO ₃ ⁻ at the in situ ¹⁴ NO ₃ ⁻ concentration. A gel probe is deployed in the sediment and is retrieved after complete diffusive equilibration between the gel and the sediment pore water. The gel is then sliced and the NH ₄ ⁺ dissolved in the gel slices is chemically converted by hypobromite to N ₂ in reaction vials. The isotopic composition of N ₂ is determined by mass spectrometry. We used the combined gel probe and isotopic labeling technique with freshwater and marine sediment cores and with sterile quartz sand with artificial gradients of ¹⁵ NH ₄ ⁺ . The results were compared to the NH ₄ ⁺ microsensor profiles measured in freshwater sediment and quartz sand and to the N ₂ O microsensor profiles measured in acetylene-amended sediments to trace denitrification.